1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method suitable for copiers, facsimile machines, laser printers, direct digital plate makers and the like.
2. Description of the Related Art
Electrophotographic photoconductors (hereinafter sometimes referred to as “photoconductor,” “latent electrostatic image bearing member” or “image bearing member”) for use in an electrophotographic image forming apparatus applicable to copiers and laser printers and the like were predominantly inorganic photoconductors made of such material as selenium, zinc oxide or cadmium sulfide. However, organic photoconductors (OPC) that are more advantageous over such inorganic photoconductors have now found many applications in view of their reduced loads on the global environment, reduced costs, and increased flexibility in design.
These organic photoconductors are broadly categorized into two types: single layer photoconductors in which at least a charge generating material, an electron transporting material and a binder resin are contained in a photosensitive layer composed of a single layer on an electric-conductive substrate; and stacked layer photoconductors prepared by stacking a charge generating layer mainly composed of a charge generating material and a charge transporting layer mainly composed of a charge transporting material.
In recent years, in view of a greater degree of freedom in design, separated-function stacked layer photoconductors have been used in a higher proportion. However, these photoconductors have been now thought to be unsuitable in forming a high-quality image because of poor productivity resulting from many coating steps and a problem in which charge occurrence in a charge generating layer moves into a charge transporting layer, during which the charge is dispersed to decrease the resolution dot density.
On the other hand, manufacture of a single layer photoconductor requires fewer coating steps and film deposition steps because of its simple layer configuration and therefore excellent in productivity. In addition, since a charge generating material is dispersed into a photosensitive layer, charge can be generated in the vicinity of the surface layer and dispersed to a lower extent, which is advantageous in realizing a higher resolution dot density. Further, these photoconductors are smaller in sensitivity variation resulting from friction. With these facts taken into account, the single layer photoconductors are favorably constituted in obtaining an image forming apparatus for realizing a stable and high-quality image, which is a major demand as an image forming apparatus nowadays.
However, due to a fact that a high quality image is realized by using the photoconductors constituted with such a single layer, there is often reported a case where in an image forming apparatus for forming images at a high density, image degradation, or “afterimage,” is found as a side effect when the apparatus is repeatedly used. It is, therefore, impossible to obtain a high-quality image output stably for a prolonged period of time as now demanded.
Here, an explanation will be made for the phenomenon of afterimages. In an image forming apparatus based on an electrophotographic process, for example, when a half-tone image is printed out after an image distinctly different in brightness, as shown in FIG. 1, there is a case where a printed-out image pattern may stand out before the half-tone image in an image at which the half-tone image should otherwise form a uniform image. FIG. 2 shows the pattern diagram thereof. This image degradation is called “positive afterimage” or “positive ghost.” Particularly, in an image forming apparatus based on a high-quality full-color electrophotographic process, the image degradation must be avoided. On the contrary, an image degradation in which a previously printed image pattern is recognized thinly at a half-tone image portion is called “negative afterimage” or “negative ghost.” In this case as well, the image degradation must be avoided. FIG. 3 shows the pattern diagram thereof.
The phenomenon of afterimages is thought to occur through several mechanisms, one of which is, for example, interpreted as occurrence due to variation in electric potential on the surface of photoconductor as described in Japanese Patent Application Laid-Open (JP-A) No. 11-133825. For explaining this interpretation, FIG. 4 graphically shows variation in electric potential on the surface of photoconductor at individual steps of latent image formation, development and post-transfer.
In this instance, upon formation of a latent image given in FIG. 4A, after the surface of the photoconductor is uniformly charged at −700V, image information is exposed (the arrow indicates exposed sites). An exposure portion is set to approximately 0V in electric potential. Then, at the time of development given in FIG. 4B, depending on a difference between the electric potential on development and the electric potential on the surface of the photoconductor, toner is coated on the surface of photoconductor to effect development. Then, at the time of transfer, printer paper is positively charged to transfer a toner image from the photoconductor to the print paper. As shown in FIG. 4C, the photoconductor is given a reverse bias by a transfer unit, and an electric potential on the surface of the photoconductor after transfer is, as a whole, shifted to a positively charged direction. An electric potential at the exposure portion exceeds 0V, and the polarity is finally reversed to result in a positive electric potential (which is indicated as +10V in the drawing). As a matter of course, the same principle is applicable to a case where the charged potential is opposite because a positive charge is switched to a negative charge or vice versa.
Upon repeated occurrence of this phenomenon, the surface electric potential of the photoconductor at a positively charged portion is accordingly charged positively in electric potential, even if the surface of photoconductor is uniformly charged negatively by using a charge unit before image exposure. As a result, a portion shifted to a positively charged direction is greater in difference in development potential than other portions, thereby an apparent intensification takes place to result in the formation of a thick toner image. This portion is identified as a positive afterimage.
As described in JP-A No. 2002-123067, for example, an afterimage will occur in a process in which an image density is processed depending on the presence or absence of dots (in a binary manner) as with a printing process widely used in an inkjet printer. Although to a minimal extent, illumination distribution is found at a beam spot into which a dot configuration is written. Therefore, upon radiation of the beam spot at a portion at which the charged electric potential is shifted positively, the surface electric potential is offset to a low electric potential, by which a developable dot border portion is expanded to result in an enlarged dot diameter.
Thus an excessively enlarged dot image is perceived to be thick when viewed as a whole image, also resulting in an image on which a positive afterimage is identified. In this instance, for example, an extent of the afterimage is perceived more intensively as an image is output at a higher resolution dot density of 1200 dpi in place of 600 dpi. Thus, this problem is further magnified when an image forming apparatus based on an electrophotographic process is made higher in resolution dot density.
A cause of the variation in surface electric potential of a photoconductor may be mainly derived from accumulation of a space charge inside a photosensitive layer as described, for example, in JP-A No. 10-177261. Therefore, in order to eliminate an afterimage, required is a unit for preventing the accumulation of a space charge.
Hereinafter, an explanation will be made for related arts of the prevention of afterimages.
(1) Improvement in Surface Layer of Photoconductor
For example, JP-A No. 10-115946 has made such a proposal that a polyallylate resin is contained in the surface layer of a photoconductor and also the dielectric constant is established to be 2.3 or more. The effect has been confirmed by referring to an example, although an explanation about a mechanism for realizing the effect is omitted due to the fact that the evaluation of the effect is in progress (paragraph number [0038]). A similar proposal is found in JP-A No. 11-184135 in which an azo pigment is contained in a photosensitive layer and a polyallylate resin is also contained in the surface layer of the photoconductor. According to this proposal, the polyallylate resin is highly crystalline, which is estimated to orient an electron transporting material to some extent due to the characteristics. The orientation may be combined with a specific charge generating material (azo pigment) to lower the barrier of an injection boundary surface, thus resulting in decrease in photo memory (paragraph number [0036]).
Further, JP-A No. 10-177263 has made such a proposal that in an image forming apparatus based on electrophotographic process and equipped with an intermediate transfer member, bisphenol polycarbonate is contained in the surface layer of a stacked-layer-structure electrophotographic photoconductor with a charge generating layer containing a phthalocyanine compound. The effect has been confirmed by referring to examples, although no explanation is made for a mechanism for providing the effect. The effect is thought to be derived from a material selected.
Further, JP-A No. 10-177264 has made such a proposal that in an image forming apparatus based on the electrophotographic process and equipped with an intermediate transfer member, an electron transporting material made up of macromolecular polymers is contained in the surface layer of a stacked-layer-structure electrophotographic photoconductor with a charge generating layer containing a phthalocyanine compound. The effect has been confirmed by referring to examples, although no explanation is made for a mechanism for providing the effect. The effect is thought to be derived from a material selected.
Further, JP-A No. 10-177269 has made such a proposal that in an image forming apparatus based on the electrophotographic process and equipped with an intermediate transfer member, a surface protective layer, which is either insulative or semi-electric conductive due to the content of a resistance adjusting material, is installed on a stacked-layer-structure electrophotographic photoconductor with a charge generating layer containing a phthalocyanine compound. The effect has been confirmed by referring to examples, although no explanation is made for a mechanism for providing the effect. The effect is thought to be derived from a material selected.
Still further, JP-A No. 2000-147803 has made such a proposal that a copolymerized polycarbonate of bisphenol A with a specific arylene group is used in a photoconductor surface layer such as a charge transporting layer to prevent the reverse polarity charge from being injected from the surface layer.
Further, JP-A No. 2001-235889 has made such a proposal that surface-treated metal oxide particles, an alcohol-soluble resin and an alcohol-soluble charge transporting material are contained as a surface-layer constituting material. It has been pointed out in this proposal that a thermoplastic resin is insufficient in strength and not suitable as a binder resin for the surface layer and a solvent for dissolving it on coating must be such a solvent that can easily dissolve the resin, thereby eliminating a method for dissolving a photosensitive layer (paragraph number [0009]). Although an explanation about a mechanism for the effect is not made, it is estimated by referring to the description of examples that an alcohol-soluble charge transporting material is used as a combination of these materials, thus making it possible to prevent the occurrence of afterimages.
Further, JP-A No. 2002-6528 has made such a proposal that in an electrophotographic photoconductor having a photosensitive layer and a protective layer, at least one of an alkaline metal element and an earth metal element is contained in the protective layer. This proposal is considered to be a unit in which these elements are contained in the protective layer, thereby giving ion conductivity to concurrently solve problems such as a decrease in durability and accumulation of remaining electric potential. This proposal has pointed out that an electron transporting material is contained in a protective layer, thereby making it possible to decrease the remaining electric potential, but also the durability results in a disadvantage of an increased friction amount (paragraph number [0019]).
(2) Improvement of Photosensitive Layer
For example, JP-A No. 2000-75521 has made such a proposal that at least one type of compound selected from chlorogallium phthalocyanine compounds and hydroxyl gallium phthalocyanine compounds is contained in an electron transporting is material, which is contained in an electrophotographic photoconductor, and also at least one type of specific compound having a hydrazone skeleton is contained as an electron transporting material. This proposal has described that a more favorable combination is found between a charge generating material and an electron transporting material between which charge is delivered, and if these substances are favorably combined, there is attained improvement in transfer memory and photo memory (paragraph numbers [0017] to [0021]). It is now difficult to estimate certain rules on whether they are favorably combined or not.
Further, JP-A No. 2000-105478 has made such a proposal that in an image forming apparatus based on the electrophotographic process which radiates short-wavelength semiconductor laser diode light of 380 nm to 500 nm into a photoconductor, an azo pigment is contained in a photosensitive layer. Although no explanation has been made for a mechanism for providing the effect, it has been confirmed by referring to the example that most of the azo pigments are lower in photo memory than a type titanyl phthalocyanine.
Further, JP-A No. 2001-305762 has made such a proposal that in an electrophotographic photoconductor containing a charge generating material and an electron transporting material, the electron transporting material contains a substance greater than 70 Å in polarizability, which is calculated by structural optimization based on semi-empirical molecular activation calculation in which PM3 parameters are used and also smaller than 1.8 D in calculation value of dipole moment, and also contains a compound having a wavelength at which the transmittance of 50% is found on the wavelength side longer than the wavelength at which the electron transporting material exhibits the transmittance of 50%. This proposal has discussed that the latter compound absorbs redundant light radiated into a photoconductor and, for this reason, there is improved the photo memory property (paragraph number [0071]).
(3) Improvement of Charge Transporting Layer
For example, JP-A No. 7-92701 has made such a proposal that in a stacked layer photoconductor, oxytitanium phthalocyanine is contained in a charge generating layer, two or more types of charge transporting materials are contained in a charge transporting layer, and individual charge transporting materials are set to be within 0.04V in oxidation potential difference. Although an ambiguous explanation has been made about a mechanism for providing the effect, the charge transporting materials are thought to be made equal in energy level, by which a smooth hopping can be given to charge carriers between the charge transporting materials, and the charge transporting materials are decreased in trapping, by which an absolute quantity of electrons excited due to antipolar charging by a transfer unit is decreased to prevent the occurrence of afterimages (paragraph numbers [0021] to [0022]).
Further, JP-A No. 8-152721 has made such a proposal that in an electrophotographic photoconductor loaded on a back-face exposure high-speed electrophotographic process (time from an exposure unit to a developing unit is about 10 msec to 150 msec), the charge mobility of a charge transporting layer is set to give 1×10−6 cm2/V·sec or more at the electric field strength of 2×106 V/cm. It has been pointed out that a slow dynamic sensitivity of a photoconductor will result in a failure in complete formation of a latent image before the development and repeated use of the photoconductor will increase an afterimage. There is proposed a unit in which the above disadvantages are eliminated, thereby securing characteristics of the dynamic sensitivity to prevent the occurrence of an afterimage (paragraph numbers [0010], [0043] to [0044]).
Further, JP-A No. 10-177262 has made such a proposal that in an image forming apparatus based on the electrophotographic process and equipped with an intermediate transfer member, an electron transporting material selected from triphenylamine compounds and N,N,N′, N′-tetraphenyl benzidine compounds is contained in the charge transporting layer of a stacked-layer-structure electrophotographic photoconductor having a charge generating layer containing a phthalocyanine compound. Although no explanation has been made about a mechanism for providing the effect, the effect has been confirmed by referring to examples, and the effect is likely to be derived from a selected material.
(4) Improvement of Charge Generating Layer
For example, JP-A No. 6-313972 has proposed such a unit that a charge generating layer is made thick as much as 0.25 μm or more, or the content of a charge generating material in the charge generating layer is increased to as much as 50% by mass or more, by which the layer is trapped for a charge to a greater extent, and a ghost is consequently made less conspicuous.
Further, JP-A No. 10-69104 has made such a proposal that in a stacked-layer-structure electrophotographic photoconductor, a triaryl amine compound having a xylyl group is contained in a charge generating layer. This proposal has described that there is formed a carrier transport barrier on a boundary surface between the charge generating layer and the charge transporting layer at which a charge is trapped. Since the thus trapped carriers act to decrease a space electrical field in the charge generating layer, a half-tone image portion is not decreased in electric potential to result in the formation of an afterimage at this portion. Therefore, a charge transport agent (xylyl group-containing triaryl amine compound) is mixed in the charge generating layer, thereby generated carriers are smoothly injected into the charge transport agent and moved to the charge transporting layer. As a result, the thus trapped carriers are prevented from accumulation to decrease the occurrence of afterimages (paragraph numbers [0011] to [0012]).
Still further, JP-A No. 10-186696 has made such a proposal that in an electrophotographic photoconductor having at least a photosensitive layer and a surface protective layer in this order on the electric-conductive substrate, oxytitanium phthalocyanine, which has a strong peak at the diffraction angle (2θ±0.2°) of 9.5°, 24.1° and 27.3° on X-ray diffraction of CuKα characteristics, as a charge generating material, is contained in the photosensitive layer. Although no explanation has been made about a mechanism for providing the effect, the effect has been confirmed by referring to examples and the effect is likely to be derived from a selected material.
Further, JP-A No. 2002-107972 has made such a proposal that a butyral resin constituted with hydroxy gallium phthalocyanine, an acetalization portion (binder resin), an acetyl group portion and a hydroxyl group portion, which is 62 mole percent or more in butyralization degree, 2.0×105 or more in mass average molecular weight and 5.0×104 or more in number average molecular weight, is contained as a material constituting a charge generating layer. It is estimated that a decreased quantity of photo carriers remaining on the photosensitive layer results in improvement of afterimages due to the effect of the butyral resin having the above-described specific constitution (for example, influence of the number of hydroxyl groups).
(5) Regulation of Matching Charge Generating Layer with Charge Transporting Layer
For example, JP-A No. 7-43920 has made such a proposal that in a stacked-layer-structure electrophotographic photoconductor, a specific azo pigment is contained in a charge generating layer, and an electron transporting material having a fluorene skeleton is also contained in a charge transporting layer. Although no explanation has been made about a mechanism for providing the effect, the effect of inhibiting light-induced fatigue has been confirmed by referring to examples and the effect is likely to be derived from a selected material.
Further, JP-A No. 9-211876 has made such a proposal that in a photoconductor which exhibits negative-polar high gamma characteristics, there is provided a stacked layer constitution in which a charge generating layer, which contains a phthalocyanine compound, and a P-type charge transporting layer are formed on an electric-conductive substrate, and a material selected from inorganic P-type semiconductors, t-Se fine particles and charge transport polymers is used in the P-type charge transporting layer. This proposal has described that there is a feature that the P-type charge transporting layer is free of positive-hole transporting molecules, thereby positive-hole transporting molecules are prevented from being dispersed into the charge generating layer, and trap by the phthalocyanine pigment is prevented to decrease the occurrence of afterimages (paragraph numbers [0003] and [0012]).
(6) Improvement of Under Layer
For example, JP-A No. 8-22136 has made such a proposal that an under layer prepared by using a silane coupling agent and an inorganic pigment is installed on an electrophotographic photoconductor. As a result, a charge which should flow to a substrate (substrate) will flow smoothly to cause no afterimage (paragraph number [0017]).
Further, JP-A No. 11-184127 has made such a proposal that in a photoconductor having an under layer (intermediate layer), polyimide structure resins with a specific polyamic acid structure or a polyamic acid ester structure and a specific-structure and a cyanoethyl-group resin are contained in the under layer. Although no explanation has been made in this proposal about a mechanism for providing the effect, the effect of inhibiting light-induced fatigue has been confirmed by referring to examples, and the effect is likely to be derived from a selected material.
Further, JP-A No. 2000-112162 has described that a cross-linking resin, the resistance value of which is less vulnerable to change in external humidity, is used in an under layer (intermediate layer) (paragraph number [0004]). The above-described JP-A No. 2000-112162 has described various proposals in reducing the occurrence of afterimages, that is, an example in which polycyclic quinone, perylene and others are contained in the under layer (JP-A No. 8-146639), an example in which a metallocen compound, an electron withdrawing compound and a melamine resin are used (JP-A No. 10-73942), an example in which fine particles of a metal oxide and a silane coupling agent are used (JP-A No. 2002-107972), and an example in which fine particles of a metal oxide surface-treated by a silane coupling agent are used (JP-A No. 9-258469).
Still further, in the case of a highly-sensitive electrophotographic photoconductor in which oxytitanium phthalocyanine is used in a charge generating layer, it has been pointed out that the high sensitivity of the photoconductor has led to a greater absolute number of excited molecules and generated carriers, thereby excited seeds, electrons and holes, which do not undergo charge separation in an electrophotographic process of repeating charge and exposure, are more likely to remain on the photoconductor (paragraph number [0010]).
Further, JP-A No. 2000-112162 has made such a proposal that a polyamide resin, a zirconium compound or a polyamide resin, and a diketone compound such as zirconium alkoxide and acetyl acetone are contained as an under layer constituting material. Similarly, JP-A No. 2001-51438 has made such a proposal that a cellulose resin is used as a resin for the under layer, and also a zirconium compound or zirconium alkoxide and a diketone compound are contained.
Further, JP-A No. 2001-305763 has made such a proposal that in an electrophotographic photoconductor, which contains an under layer, a charge generating material and an electron transporting material, the electron transporting material is a substance greater than 70 Å in value of polarizability calculated by structural optimization based on semi-empirical molecular activation calculation in which PM3 parameters are used and also smaller than 1.8 D in calculation value of dipole moment or a specific arylamine compound, and a polyamide having titanium oxide particles coated with an organic silicon compound and a specific-structured diamine composition as its constituents is contained in the under layer. JP-A No. 2001-305763 has confirmed that the under layer is installed to improve characteristics of photo memory. The under layer is installed as this mechanism, by which carriers remaining on a photosensitive layer may be allowed to move out easily (paragraph number [0075]).
Further, JP-A No. 2002-107983 has made such a proposal that in a stacked layer photoconductor having an under layer (intermediate layer), the volume resistivity of the under layer is set to be from 1010 to 1012 Ωcm, the thickness of a charge transporting layer is set to be 18 μm or less, and a charge eliminating unit is omitted. In this proposal, the antistatic unit (charge eliminating light) is omitted, thereby preventing light-induced fatigue of the photoconductor, and the under layer is regulated for resistance, thereby controlling the charge injection from a substrate to the photoconductor, thus making it possible to prevent the accumulation of a space charge (paragraph numbers [0005], [0025] to [0029].
(7) Formulation of Additives
For example, JP-A No. 10-177261 has made such a proposal that in an image forming apparatus based on electrophotographic process, which has an intermediate transfer member, at least a hindered phenol structure unit is contained in the surface layer of a stacked-layer-structure electrophotographic photoconductor having a charge generating layer containing a phthalocyanine compound. Although no explanation has been made about a mechanism for providing the effect, the effect has been confirmed by referring to examples. The effect is thought to be derived from a selected material.
Further, JP-A No. 2000-292946 has made such a proposal that a dithiobenzyl compound is contained in a charge generating layer in which a phthalocyanine pigment is used. Although an explanation about a mechanism for providing the effect has been omitted, examples have shown the improvement in accumulated photo memory and positive ghost.
(8) Modification of Electrophotographic Process
For example, JP-A No. 7-13374 has made such a proposal that a photoconductor is used by being charged or allowed to stand, with the polarity being reversed (plus) to a normal charge, under predetermined conditions. In the case of a photoconductor having a high-sensitive charge transporting layer, largely found are light-induced charge carriers generated on exposure. The light-induced charge carriers generate the same number of electrons as that of the holes injected into a charge transporting layer. However, if the electrons fail in quickly moving to a substrate, they remain on the charge generating layer to result in the formation of afterimages. Therefore, a positive charge is intentionally conducted to inject the electrons from the substrate, thus retaining an electron trap inside the charge generating layer. The proposal is considered as a unit in which on exposure of the photoconductor, with this state kept, there is found a smaller difference in electron trap between an exposure portion and a non-exposure portion, thus making a ghost image less conspicuous (paragraph numbers [0016] to [0022]).
Further, JP-A No. 7-44065 has proposed a unit in which alternating-current overlapped direct current electricity is applied to the substrate of a photoconductor. This unit is interpreted as a unit for applying electrons trapped on a charge generating layer to the substrate in a reverse bias manner so that they are allowed to move out. It has been described that alternating current is overlapped for the purpose of increasing a quantity of current flow to accelerate the reversely-charged bias effect (paragraph numbers [0019] to [0021]).
Still further, JP-A No. 10-123802 has made such a proposal that a stacked-layer structure electrophotographic photoconductor with a charge generating layer containing a phthalocyanine compound is subjected to charge (other than primary charge), then, to static elimination by light, primary charge is conducted from the time when a portion of the electrophotographic photoconductor, which is initially subjected to the primary charge, advances into a position opposing a unit of conducting the primary charge, thereby making it possible to form an image in a state that a space charge inside the photoconductor is liberated and eliminated and also to prevent the occurrence of afterimages at an initial stage of image formation (paragraph numbers [0012], [0020]).
Further, JP-A No. 10-123855 has made such a proposal that a control unit for controlling at a constant level a transfer current flowing into the photoconductor from a transfer unit is installed on a stacked-layer-structure electrophotographic photoconductor with a charge generating layer containing a phthalocyanine compound. According to this proposal, an afterimage occurs, depending on the transfer current, and a negative afterimage appears more clearly as the transfer current is made larger. This is estimated to be due to a mechanism that holes (positive holes) are injected into a non-exposure portion (no image portion) of the photoconductor on transfer, the holes are trapped on the boundary surface of a charge generating layer or of a charge transporting layer on a base material side, liberated at the time of a next charge process, and increased in dark decay (apparent intensification), thereby generating a negative afterimage. Therefore, values of the transfer current are controlled at a constant level, by which charge injected into the photoconductor can be constantly controlled to result in the prevention of an afterimage (paragraph number [0012]).
Further, JP-A No. 2000-231246 has proposed a unit of regulating writing light wavelength or antistatic light wavelength by referring to action spectra of a light memory ratio before a charge with respect to the sensitivity.
Still further, JP-A No. 10-123856 has made such a proposal that a stacked-layer-structure electrophotographic photoconductor with a charge generating layer containing a phthalocyanine compound is subjected to exposure before transfer, by which the charged electric potential at a non-exposure portion can be reduced to one-third of that before exposure, thereby preventing the occurrence of an afterimage. No detailed explanation has been made about a mechanism for providing the effect. However, exposure before transfer would decrease a gap difference in electric potential between an exposure portion and a non-exposure portion, thus making an afterimage undistinguishable.
Further, JP-A No. 10-246997 has made such a proposal that in an image forming apparatus based on electrophotographic process which uses an electrophotographic photoconductor with a protective layer containing a photosensitive layer and a photocurable resin (acryl resin), a humidity sensor is installed near the surface of the electrophotographic photoconductor. In this proposal, the humidity sensor is to control current values of an alternating current component applied to a charge member. This proposal has described a mechanism for reducing a rough image and a blurred image on installation of the humidity sensor but has not described an effect of the mechanism on reduction in photo memory. Examples have confirmed the reduction in photo memory on installation of the humidity sensor.
Further, JP-A No. 2001-117244 has made such a proposal that the half time of a charged electric potential of a photoconductor on exposure calculated by the zerographic TOF method is reduced to 1/10 or less the time from an exposure unit to a developing unit in the image forming apparatus based on electrophotographic process (hereinafter, sometimes referred to as “exposure-development time”) as measures for preventing the occurrence of ghost images in an S-letter shaped photoconductor.
Further, as described in the above Section of “Improvement of under layer,” in JP-A No. 2002-107983, there is proposed a method in which a charge eliminating unit (charge eliminating light) is omitted, thereby preventing the light-induced fatigue of a photoconductor.
Further, JP-A No. 2002-123067 has made such a proposal that if time from charge to exposure is given T; charged electric potential on the surface of a photoconductor, VH; electric potential which is dark-decayed until 10 T after being charged, V1; and electric potential which is dark-decayed until 10 T after being re-charged following the passage of charge and image exposure, V2; the relationship of |(V1−V2)/VH|<0.020 is satisfied. This proposal has described such an example that the process speed is increased to shorten the dark-decay time or the charged electric potential is decreased as an actual unit.
In order to prevent the occurrence of afterimages, we attempted to put into practical use conventional technologies described in the documents so far explained, finding that these technologies are only insufficiently applicable to an electrophotographic photoconductor intended for prints high in durability, speed and quality as well as to an image forming apparatus based on an electrophotographic process in which the above-described electrophotographic photoconductor is used. Therefore, these technologies are unable to solve problems.